When an interface doesn't have a layer 2 address in6_get_soii_ifid()

When an interface doesn't have a layer 2 address in6_get_soii_ifid()
failes and then later on a in in6_get_ifid() a layer 2 address is
"borrowed" from from another interface.

Do the "borrowing" in in6_get_soii_ifid(), too so that semantically
opaque interface identifiers work for these kind of interfaces, too.

OK phessler, benno